Molecular cloning and expression analysis of the IgM heavy chain gene in largefin longbarbel catfish(Mystus macropterus)

Full cDNA sequence of a secretory IgM(sIgM)gene in largefin longbarbel catfish(Mystus macropterus Bleeker)was cloned using RACE-PCR and RT-PCR methods.It has 1 992 nucleotides,including 5′UTR of 53 nucleotides,3′UTR of 226 nucleotides and an open reading frame with 1 713 nucleotides encoding a 570 amino acid peptide.The deduced amino acid sequence in M.macropterus contains 4 constant regions(CH)and a variable domain(VH)which consists of 4 frame regions (FRs)and 3 complementary determining regions(CDRs).The IgM comparison in seven teleost species showed that IgM in M. macropterus shared the highest identity(54.3%)with that in Ictalurus punctatus, and the lowest identity(24.6%)with that in Epinephelus coioides, and that the conserved cysteines and tryptophans existed in all fishes involved.Phylogenetic tree based on some teleost Ig heavy chain amino acids suggested IgM in M.macropterus was clustered closely with that of I.punctatus. Fluorescent real-time PCR showed that IgM mRNA expression of M. macropterus was mainly detected in blood cells,spleen,kidney and head kidney,and increased significantly in blood cells,spleen,head kidney between day 1 and day 10 after injection of Aeromonas hydrophila.